A surface-mounting type light emitting device with a light emitting element such as a light emission diode (LED) or a laser diode (LD) mounted on a substrate is conventionally well known. The light-emitting device has been used in lighting equipment, a backlight for display screen, an automotive illuminant, a display illuminant, and an auxiliary illuminant for motion pictures, as well as for other general consumer illuminants. In those applications, a device with a configuration comprising both anodes and cathodes mounted on the same surface is widely used in order to enhance its light extraction efficiency. A widely proposed light-emitting device has a configuration comprising chips mounted, by a flip-chip method, on an insulating substrate, on which wiring patterns are formed.
One example of the light emitting device is manufactured by applying a solder paste at specific positions on a pair of electrodes formed on a insulating substrate, mounting LED elements by flip-chip mounting in order to bond the LED elements, and melting the solder paste by reflowing in order to make the electrodes and the LED elements electrically connected to each other. If the pair of the electrodes formed on the insulating substrate of the light emitting device are formed into a shape identical to that of the electrodes on the side of the LED elements, when the LED elements are mounted by pressing themselves against the electrodes, a portion of the solder paste applied on the insulating substrate bulges out around the electrode, comes off the electrodes during the reflowing process, and forms a solder ball.
Therefore, conventional light emitting devices are known to have such a configuration described in JP1994-12648A (“JP '648”) to prevent the solder ball generation in the light emitting device production process. That is, JP '648 discloses a configuration for preventing solder ball generation by providing a redundant reflow pad along the outer circumference of the LED element.
Further, JP2003-264267A (“JP '267”) discloses another configuration which ensures self-alignment by setting a length and a width of a rectangular die pad at 0.5-1.5 times of those of the rectangular light emitting diode of the light emitting device. JP '267 also discloses a configuration in which extended sections extending from each of the four sides of the light emitting diode are provided when a light emitting diode is mounted on the die pad at the side of the insulating substrate. The extended sections are able to lower the height of the molten solder deposited on the surface of the die pad so that self alignment by the surface tension of the molten soldier is ensured.
Despite the above-mentioned prior art conventional light emitting devices have the following problems.
In the configuration of the light emitting device of JP '648, the shape of the redundant reflow pad, which is formed to be larger than the electrode section of the element, is not suitable for self-alignment performance. Therefore, improper self-alignment is a problem, which causes an improper positioning of the mounted light emitting element at the time of the reflowing process.
Although the configuration of the light emitting device of JP '267 may enable self-alignment during the reflowing process, in this document, the mechanism of self-alignment assumes the shape of the die pad as a single unit. Therefore, when the device comprises separate die pads for one element such as a light emitting diode, in other words, when the configuration of the device comprises multiple electrodes on the side of a substrate, such as a first substrate electrode and a second substrate electrode mounted at separate positions on the substrate, the configuration is unsatisfactory for performing self-alignment while preventing the solder ball generation.
In particular, when the electrodes formed on the substrate, at the time of flip-chip mounting of the element, are a first electrode and a second electrode, corresponding to a P-type electrode and a N-type electrode, and are separated from each other, the molten solder may converge towards the second electrode from the first electrode. Therefore, the converging molten solder forms an excessive connecting thickness of the second electrode, and may lift up some portion of the molten solder around the first electrode, generating voids in the molten solder near the first electrode. Therefore, a method for prevention of the voids around the first electrode in a light emitting device is desired.
The objective of the present invention made in view of the above-mentioned problems is to provide a light emitting device that secures self-alignment, and prevents the generation of solder balls as well as the generation of voids in molten solder under the electrodes, even in case of electrodes formed separately from each other on a substrate having a single mounted element.